The present invention relates to an active matrix display device and, more particularly, to a technique of preventing potential fluctuation on a video signal line in dot-sequential driving.
An exemplary construction of an active matrix display device in the related art will be briefly described below with reference to FIG. 6. This active matrix display device has row gate lines G, column signal lines S, and matrix liquid crystal pixels LC disposed at intersections of the gate lines and the signal lines. Each of the liquid crystal pixels LC is driven by a thin-film transistor Tr. A V shift register (vertical scanning circuit) 101 line-sequentially scans the gate lines G and selects liquid crystal pixels LC of one row during each horizontal scanning period (1H). An H shift register (horizontal scanning circuit) 102 sequentially samples a video signal to the signal lines S within a period of 1H and writes the video signal dot-sequentially in the selected liquid crystal pixels LC of one row. More specifically, the signal lines S are connected via horizontal switches HSW to a video line and are supplied with a video signal from a signal driver 103, while the H shift register 102 sequentially outputs horizontal sampling pulses H1, H2, H3, . . . , Hn to execute on-off control of the horizontal switches HSW.
FIG. 7 shows waveforms of sampling pulses. With improvements in attaining higher precision of the active matrix display device, the sampling rate is raised to consequently cause variation of the sampling pulse width .tau..sub.H. In response to an output sampling pulse, the horizontal switch HSW corresponding thereto is turned on and off, so that a video signal from the video line is sampled and held to the corresponding signal line S. Since each signal line S has a capacitive component, charge and discharge are caused by such sampling of the video signal to eventually fluctuate the potential of the video line. With rise of the sampling rate, the sampling pulse width .tau..sub.H is varied as described above, so that the charge and discharge relative to each signal line S are not retained constant to thereby fluctuate the potential of the video line. And this phenomenon appears as a fixed pattern of vertical streaks to consequently bring about a problem that the definition of the displayed picture is extremely impaired. On a display conforming with the normal NTSC standard, the sampling rate is relatively low and a next sampling pulse falls after the video line potential begins to fluctuate, so that the preceding signal line is not affected harmfully and therefore none of fixed pattern of vertical streaks appears. However, in high-definition (HD) TV or double-speed NTSC, the sampling rate is extremely raised and it becomes difficult to achieve effective suppression of the potential fluctuation on the video line. Sampling pulses are produced in an H shift register consisting generally of thin-film transistors (TFTs). In a TFT, the mobility is lower than in a monocrystal transistor and variations of the physical constants are larger. Therefore it is difficult to precisely control the sampling pulses produced in this circuit. And in addition to the variation of the sampling pulse width, there also occurs some variation in the on-resistance of each horizontal switch HSW. Consequently the charge-discharge characteristics of the signal line S are varied to cause fluctuation of the video line potential, which is superposed on the actual video signal to eventually become vertical streaks, hence impairing the definition of the displayed picture conspicuously.